Two-dimensional (2D) radio frequency multipole ion traps have been used for several years for the study of spectroscopic and other physical properties of ions. The earliest application of 2D multipole ion traps in mass spectrometry involved the use of the collision cell of a triple quadrupole instrument for studying ion-molecule reactions. More recently, multipole ion traps have been used in mass spectrometers as part of hybrid systems including Fourier transform ion cyclotron resonance (FTICR), time-of-flight (TOF), and standard three-dimensional (3D) ion trap mass spectrometers.
Syka and Fies have described the theoretical advantages of 2D versus 3D quadrupole ion traps for Fourier transform mass spectrometry (U.S. Pat. No. 4,755,670). These advantages include reduced space charge effects due to the increased ion storage volume, and enhanced sensitivity for externally injected ions due to higher trapping efficiencies. Bier and Syka described several forms of linear and circular 2D ion traps with large ion capacity to be used as mass spectrometers (U.S. Pat. No. 5,420,425) using the mass selective instability mode of operation similar to that used in all commercial three-dimensional (3D) quadrupole ion trap instruments.
A linear ion trap includes two pairs of electrodes or rods, which contain ions by utilizing an RF quadrupole trapping field in two dimensions, while a non-quadrupole DC trapping field is used in the third dimension. Simple plate lenses at the ends of a quadrupole structure can provide the DC trapping field. This approach, however, allows ions which enter the region close to the plate lenses to be exposed to substantial fringe fields due to the ending of the RF quadrupole field. These non-linear fringe fields can cause radial or axial excitation which can result in loss of ions. In addition, the fringe fields can cause shifting of the ions frequency of motion in both the radial and axial dimensions.
An improved electrode structure of a linear quadrupole ion trap 11, which is known from the prior art, is shown in FIG. 1. The quadrupole structure includes two pairs of opposing electrodes or rods, the rods having a hyperbolic profile to substantially match the equipotential contours of the quadrupole RF fields desired within the structure. Each of the rods is cut into a main or central section and front and back sections. The two end sections differ in DC potential from the central section to form a “potential well” in the center to constrain ions axially. An aperture or slot 12 allows trapped ions to be selectively resonantly ejected in a direction orthogonal to the axis in response to AC dipolar or quadrupolar electric fields applied to the rod pair containing the slotted electrode. In this figure, as per convention, the rod pairs are aligned with the x and y axes and are therefore denoted as the X and Y rod pairs.
FIGS. 2a–2c schematically show the voltages needed to operate this linear ion trap as a mass spectrometer. These voltages include three separate DC voltages, DC1, DC2 and DC3, (typical range of 0 to +/−100 volts) applied to the electrodes of the front, center, and back sections to produce the injection and axial trapping fields (FIG. 2a), two phases of primary RF voltage (typical value of +/−5KV, with frequencies in the 1 MHz range) applied to opposite rod pairs of the three sections to produce the radial trapping fields (FIG. 2b), and, two phases of the AC resonance excitation voltage (typical range of +/−100V, 5–500 kHz) applied to the pair of electrodes which include the ejection slot(s) for isolation, activation, and ejection of ions (FIG. 2c).
When using a linear ion trap operated in the resonance ejection mass instability mode, the mass spectra and resolution are controlled by many of the same processes in the linear ion trap as in a three-dimensional ion trap such as described in U.S. Pat. Nos. 4,540,884 and 4,736,101. However, unlike most three-dimensional ion traps where the trap structure does not require high mechanical tolerances, the performance of a two-dimensional ion trap is more susceptible to mechanical errors. In a three-dimensional ion trap, all of the ions occupy a spherical or ellipsoidal space at the center of the trap typically of a cloud size of 1 mm in diameter. The ions in a two-dimensional ion trap, however, are spread out along a substantial fraction of the entire length of the trap in the axial direction which can be several centimeters or more. Therefore, one could imagine that if the quadrupole rods are not completely parallel, then ions at different axial positions within the trap will experience a slightly different field strength. This variation in field strength experienced will in turn cause ejection times during mass analysis which are dependent on the ions axial position. The result is increased overall peak widths and degraded resolution. In such a device, if the axial spread of the ion cloud could be reduced using, for example, higher end-section DC voltages, then a smaller variation of q values would be obtained and better resolution would result. This could compromise ion storage volume or space charge capacity for this device, but could make a distorted device into a usable mass spectrometer.
Other parameters also contribute to the overall performance of the linear trap as a mass spectrometer. When using a mass selective instability scan in a linear ion trap, the ions are ejected from the trap in a radial direction. Some researchers have ejected ions between two of the quadrupole rods. However, due to high field gradients loss of ions is substantial. The more efficient way is to eject the ions through a rod by introducing a slot in the rod. For the linear ion trap, the preferred operation is a slot cut along the length of the rod. When a slot (or slots) is cut into one or more of the linear ion trap electrodes to allow ions to be ejected from the device, the electric fields are degraded from the theoretical quadrupole field and therefore the presence of this slot can impact several important performance factors. Consequently, the characteristics of this slot are significant. It should also be noted, that distortion of the electric fields can also be caused by truncation of the hyperbolic surface of the electrodes. Similar to the effects of the slots, these effects also cause field faults and so the overall performance will depend on the combined effects of the slots and the truncation. Normally these truncation effects are small relative to the slots, however the possibility of using their interaction to optimize overall performance exists.